In industry the use of metal products manufactured by compacting and sintering metal-powder compositions is becoming increasingly widespread. A number of different products of varying shapes and thickness are being produced and the quality requirements are continuously raised at the same time as it is desired to reduce the costs. This is particularly true for P/M parts for the automotive market, which is an important market for the P/M industry and for which cost is a major driving force. Another factor of importance is the possibility of recycling scrap from the automotive industry and to consider the effect on the environment. Known alloying systems which have gained wide acceptance within this field have frequently included alloying elements such as Ni and Cu. However, nickel is a strong allergen and is also considered to have other detrimental medical effects. A problem with copper is that, during recycling of scrap used for steel manufacture, copper is accumulated. In many steel qualities copper is however not suitable and scrap without copper or with a minimum of copper would be required. Iron-based powders having low amounts of alloying elements without nickel and copper are previously known from e.g. the U.S. Pat. Nos. 4,266,974, 5,605,559, 5,666,634 and 6,348,080 (Arvidsson)
The purpose of the invention according to the U.S. Pat. No. 4,266,974 is to provide a powder satisfying the demands of high compressibility and moldability of the powder and good heat-treatment properties, such as carburising, hardenability, in the sintered body. The most important step in the production of the steel alloy powder produced according to this patent is the reduction annealing step (col. 5 line 15).
The U.S. Pat. Nos. 5,605,559 and 5,666,634 both concern steel powders including Cr, Mo and Mn. The alloy steel powder according to the U.S. Pat. No. 5,605,559 comprises, by wt %, about 0.5-2% of Cr, not greater than about 0.08% of Mn, about 0.1-0.6% of Mo, about 0.05-0.5% of V, not greater than about 0.015 of S, not greater than about 0.2% of 0, and the balance being Fe and incidental impurities. The U.S. Pat. No. 5,666,634 discloses that the effective amounts should be between 0.5 and 3% by weight of chromium, 0.1 and 2% by weight of molybdenum and at most 0.08% by weight of manganese.
A serious drawback when using the inventions disclosed in these U.S. Pat. Nos. 5,605,559 and 5,666,634 is that cheap scrap cannot be used as this scrap normally includes more than 0.08% by weight of manganese. In this context the patent U.S. Pat. No. 5,605,559 teaches that “when Mn content exceeds about 0.08 wt % oxide is produced on the surface of alloy steel powders such that compressibility is lowered and hardenability increased beyond the required level. . . . Mn content is preferably not greater than about 0.06% wt.” (col 3 47-53).This teaching is repeated in the U.S. Pat. No. 5,666,634 disclosing that “a specific treatment is used in order to reduce the Mn content to a level not larger than 0.08% by weight during the course of the steel making” (col. 3 line 40-44). Another problem is that nothing is taught about the reduction annealing and the possibility to obtain the low oxygen and carbon content in water-atomised iron powders including elements sensitive to oxidation, such as chromium, manganese. The only information given in this respect seems to be in example 1, which discloses that a final reduction has to be performed. Furthermore the U.S. Pat. No. 5,666,634 refers to a Japanese Patent Laid-open No. 4-165002 which concerns an alloy steel powder including in addition to Cr also Mn, Nb and V. This alloy powder may also include Mo in amounts above 0.5% by weight. According to the investigations referred to in the U.S. Pat. No. 5,666,634, it was found that this Cr-based alloy steel powder is disadvantageous due to the existence of the carbides and nitrides which act as sites of fracture in the sintered body.
The possibility of using powders from scrap is disclosed in the U.S. Pat. No. 6,348,080 which discloses a water-atomised, annealed iron-based powder comprising, by weight %, Cr 2.5-3.5, Mo 0.3-0.7, Mn 0.09-0.3, O<0.2, C<0.01 the balance being iron and, an amount of not more than 1%, inevitable impurities. This patent also discloses a method of preparing such a powder. Additionally the U.S. Pat. No. 6,261,514 discloses the possibility of obtaining sintered products having high tensile strength and high impact strength if powders having this composition is warm compacted and sintered at a temperature>1220° C.
The present inventors have now unexpectedly found that more narrow ranges of the alloying elements, especially chromium, will give unexpected improvements as regards the possibilities of annealing and sintering the powders in comparison with the powders disclosed in the U.S. Pat. No. 6,348,080.
Additionally, when comparing green bodies prepared from these known powders with green bodies prepared from the new powders according to the present invention it was found that the green strength of compacted bodies prepared from the new powders are distinguished by an unexpectedly high green strength. This is particularly true when die wall lubrication is used. Green strength is one of the most important physical properties of green parts. The importance of this property increases as P/M parts increase in size and geometry becomes more complex. Green strength increases with increasing compact density and is influenced by type and amount of lubricant admixed to the powder. The green strength is also influenced by the type of powder used. A high green strength is required in order to prevent compacts from cracking during the ejection from the compacting tool and prevent them from getting damaged during the handling and the transport between the press and the sintering furnace. Presently used compacts having a relatively high green strength are advantageously prepared from sponge iron powders whereas difficulties have been met as regards the preparation of compacts of atomised powders in spite of the fact that an atomised powder is more compressible and hence gives a higher green density.